Embodiments of the present invention relate generally to microactuator control and, more particularly, to active control of a microactuator.
Disk drives are widely used in computers and other electronic devices for the storage and retrieval of data. Disk drive manufacturers are constantly working to try to increase the performance of disk drives by increasing a data transfer rate of the disk drives. Two main ways to increase the data transfer rate of disk drives are to: (i) lower a seek time, which is a time required to position a head for a read or a write operation; and (ii) improve an accuracy of track following operations, which maintain a head on a track for a read or a write operation. Thus, disk drive manufacturers are very interested in new ways of reducing seek time and improving the accuracy of track following operations.
In general, related art disk drives comprise one or more disks for storing data, a coarse actuator, a microactuator, an actuator arm assembly, one or more transducers or heads, and a servo controller. Each head is operable to read data from and to write data to one or more tracks on a surface of a corresponding disk. The actuator arm assembly includes a first member connected to a base of the disk drive and a second member on which the heads are mounted. The microactuator interconnects the first member and the second member of the actuator arm assembly to provide for fine positioning of the heads. The coarse actuator controls the actuator arm assembly to move the microactuator relative to the disk and, as a consequence, provides coarse positioning of the heads.
Servo controllers of disk drives typically perform the functions of seek control and track following. To perform seek control, servo controllers generally provide for a single-stage seek trajectory in which the servo controller controls the coarse actuator to move the actuator arm assembly and, as a result, position a head over a desired track of the disk for a read or a write operation. Such a servo controller may be configured to control the movement of the coarse actuator based on a coarse actuator control loop. As part of the coarse actuator control loop, the servo controller determines a position error signal during the positioning of the head by reading servo data stored in servo sectors on the disk. The servo controller then controls the movement of the coarse actuator based on the position error signal.
In the past, the microactuator has generally not been used for seek operations. Instead, the microactuator has typically been used for track following. Servo controllers generally perform track following once a head has been positioned over a desired track after a seek operation. Such servo controllers may control the microactuator to finely position the head with respect to the desired track based on a position error signal, so that an accuracy of track following can be increased. The range of movement of the microactuator is generally much less than the range of movement of the coarse actuator, and microactuators typically have a maximum range of movement of about a few tracks in either direction. In contrast, most coarse actuators have a range of movement that allows for movement of a head across all tracks on a disk.
However, the microactuator of related art disk drives generally provides for much faster positioning of the head than is provided for by the coarse actuator. Such a difference in positioning speed is realized because the microactuator typically comprises a piezoelectric (PZT) device, or the like, that moves immediately when a current is applied, whereas the coarse actuator is typically a voice coil motor (VCM) that does not move the actuator arm assembly immediately when a current is applied due to some inertia.
Recently, there has been an effort to take advantage of the rapid movement of the microactuator to reduce seek time during short seeks that seek the head over a small number of tracks. Instead of only using the coarse actuator to perform a seek operation, it has been proposed to use both the coarse actuator and the microactuator during seek operations by having a dual stage seek trajectory, including both a stepping stage and a retracting stage. Such a dual stage seek trajectory takes advantage of the ability of the microactuator to move the head rapidly within a certain range that can cover a small number of tracks.
During the stepping stage of a dual stage seek trajectory, the microactuator moves the head to a target track on the disk very quickly. Then, once the head is over the target track, the retracting stage begins in which the microactuator moves in the opposite direction of the coarse actuator so as to keep the head over the target track while the coarse actuator moves the microactuator toward the target track. In the best case, the head may be declared on-track at the end of the stepping stage when the head reaches the target track, while both the coarse actuator and the microactuator are still moving.
The microactuator must be controlled very precisely to reduce post seek track-misregistration (TMR) when a dual stage seek trajectory is employed. Precise control of the microactuator is also important for track following operations. In order to control the microactuator, some related art disk drives have a microactuator control loop, which allows for a movement of the microactuator to be controlled in accordance with a control signal, and where a position error signal is used as feedback for the microactuator control loop. A control portion of the microactuator control loop is generally designed by assuming a certain gain for the microactuator and then setting control values for generating the control signal based on the assumption of the certain gain, where the control values remain constant during disk drive operation.
However, in reality, a gain of the microactuator may change during operation due to, for example, environmental conditions, so the microactuator will not move as precisely as expected based on such a control signal. The control of the microactuator in such related art systems will not be as precise as expected when the gain of the microactuator changes from an assumed gain that is used to set the control values to a different gain due to, for example, a change in ambient temperature. Thus, in the related art disk drives, there is still an undesirable amount of seek TMR associated with dual stage seek operations that inhibits on-track from being declared sooner. As a result, seek time is increased due to the undesirable seek TMR and the data transfer rate of the disk drives is reduced.
In light of the above-mentioned problems, there is a need for systems and methods that allow for improved control of microactuators during track following operations and during short seek operations with dual stage seek trajectories, so as to compensate for position errors due to changes in microactuator gains caused by changes in temperature, and the like.